The invention relates generally to sensors for detecting substances, and in particular to a gas sensor for monitoring the presence or quantity of a target gas in an environment or gas sample.
Chemical sensing of gases is an important technology in several fields including environmental monitoring, industrial safety, and public security. Depending on the application and target gas, different operating principles have been deployed, ranging from electrochemical, metal-oxide semiconductor, and non-dispersive infrared absorption. With the recent advancement of smartphones, wearables, and connected sensor devices, many more chemical sensing applications have emerged. In particular monitoring air quality for the health, safety and well being of consumers is receiving considerable interest. The problem is that existing technologies and sensor devices do not scale to the small size, low cost, and low power consumption that is required for these emerging applications.
One approach to a scalable gas sensor is a solid-state device based on a sensing material applied as a coating to a resonant mass transducer. This architecture can scale in size, cost, and power, but the challenge lies in the development of a sensing material that can provide sufficient sensor performance. One problem to overcome is that resonant chemical sensors can be highly sensitive to other gases including even moderate changes in relative humidity. When measuring an analyte in ambient environmental conditions, the sensor indicates not only a mass change due to the presence of a target gas, but also due to the additional adsorption of water molecules in the sensing material. While many sensing materials can be tailored to detect specific analyte molecules with low cross sensitivity, the absorption of water molecules is a more difficult problem to solve due its omnipresence and chemical activity.
Carbon dioxide (CO2) sensors are currently deployed for monitoring indoor air quality to ensure adequate ventilation and as a component of demand-controlled ventilation systems. However, current optical CO2 sensors are costly and prone to long-term drift, which requires manual recalibration about every six months. A low-cost sensor not requiring manual calibration would enable an expanded deployment of demand-controlled ventilation and indoor air quality CO2 monitoring. Past attempts to utilize adsorbent-based sensors have failed to achieve sufficient sensitivity and immunity to interference due to variations in humidity, which cause an undesired response in the sensor device. There is still a need for a simple, low cost, sensor to detect analyte that overcomes the problems of humidity and water adsorption by the sensing material.